The present invention relates to implantable medical devices, and more particularly to an implantable medical sensor that can be used to determine the oxygen content of blood. In a preferred embodiment, the sensor is housed in a miniature hermetically-sealed capsule that is embedded into an implantable pacemaker lead of a rate-responsive pacemaker.
It is known in the art to use an implantable sensor to determine the oxygen content of blood. It is also known in the art to utilize such a sensor in combination with a pacemaker in order to adjust the pacing interval or frequency of the pacemaker as a function of the oxygen content of the blood. See U.S. Pat. Nos. 4,202,339 and 4,399,820. Further, as described in U.S. Pat. No. 4,399,820 (hereafter the '820 Patent), the prior art recognizes the advantages of incorporating an oxygen sensor made from a light-emitting diode (LED) and a light receiving transistor, or phototransistor (PT), into an implantable cardiac pacing lead.
Unfortunately, the pacing system, including the lead and sensor, described in the '820 Patent, while representing a significant advance in the art at the time of its development, still exhibits some problems which have heretofore hindered a widespread clinical use of such a pacing system. For example, while the LED and PT elements used to realize the oxygen sensor of the pacing system taught in the '820 patent are advantageously embedded into the pacing lead, they are done so in a way that does not guarantee a true hermetic seal, there being at least one annular insulation layer 35 through which body fluids seep into the otherwise closed area where the LED element 32 and PT element 37 are located. Further, given the relatively large size of the annular elements used to create the pocket within which the LED and PT are located, e.g., the glass ring 39, and metal annular elements 31 or 31' and 34 or 34', the glass ring having a diameter approximately the same as the lead diameter, and given that the annular metallic elements have an outside diameter that must be firmly welded to the glass ring all around the circumference thereof, there is a relatively large weld that must not develop any leaks. This is not an easy task using conventional bonding techniques, especially given the periodic forces that are regularly placed on the lead as it moves or flexes within the heart or body.
Thus, disadvantageously the sensor taught in the '820 Patent is not easily sealed and protected from body fluids that might find their way into the lead, and even if it is initially sealed, it may not remain so with use. Needless to say, the presence of such body fluids within the lead could dramatically alter the optical properties and performance of the sensor system, as well as, the pacer output capabilities. Further, the arrangement shown in the '820 patent is very expensive, both in manufacturing time and cost. What is needed, therefore, is a more economical sealed sensor that can more readily be embedded into a pacing lead and that will remain tightly sealed throughout the life of the lead.